This invention relates to a failure determination method for an O.sub.2 sensor which detects the oxygen concentration of exhaust gas from an internal combustion engine. More particularly, it relates to a failure determination method capable of accurately detecting performance degradation, disconnection, etc. of the O.sub.2 sensor.
It is known in the art to arrange a three-way catalytic converter in an exhaust passage of an internal combustion engine installed in a motor vehicle, to remove harmful substances, such as CO, HC and NO.sub.x contained in the exhaust gas emitted from the engine. The three-way catalytic converter is advantageous in that a single catalyst can treat the three substances, i.e., CO, HC and NO.sub.x, at the same time. However, to obtain the catalytic power of the three-way catalytic converter equally for the three substances at the same time, the air-fuel ratio of a mixture supplied to the engine must be controlled to a value falling within a range (14.7.+-.0.05) of a air-fuel ratio. When carrying out air-fuel ratio feedback control of this type, information as to whether an actual air-fuel ratio of the mixture is rich or lean is required, and as typical measures, the oxygen concentration of the exhaust gas is detected as the information representing the actual air-fuel ratio. The oxygen concentration of the exhaust gas increases with an increase in the air-fuel ratio, and is detected by an O.sub.2 sensor arranged in the exhaust passage on the upper-course side of the three-way catalytic converter. The air-fuel ratio control is carried out based on the result of the comparison between the O.sub.2 sensor output representing the actual air-fuel ratio and the stoichiometric air-fuel ratio.
The O.sub.2 sensor is arranged in the exhaust passage as mentioned above, and thus is exposed to high-temperature exhaust gas. Accordingly, the performance of the O.sub.2 sensor may be degraded, or disconnection or an insulation defect may occur in the O.sub.2 sensor. If such failure occurs in the O.sub.2 sensor, the accuracy in detecting the oxygen concentration of the exhaust gas, which represents the actual air-fuel ratio, lowers, or the oxygen concentration itself cannot be detected, making it impossible to carry out proper air-fuel ratio control based on the output of the O.sub.2 sensor. In such cases, it is difficult to control the air-fuel ratio to a value close to the stoichiometric ratio and to have the three-way catalytic converter function effectively. As a result, the exhaust gas cannot be purified satisfactorily.
To eliminate these drawbacks, various methods have been proposed for determining failure of the O.sub.2 sensor. For example, in the description of prior art in Unexamined Japanese Patent Publication (KOKAI) No. 4-16757, a method is described in which degradation of the O.sub.2 sensor is detected when the voltage amplitude of the O.sub.2 sensor becomes smaller than a preset value during the air-fuel ratio control or when the control period of the air-fuel ratio control becomes greater than a preset value. In Unexamined Japanese Patent Publication No. 4-16757, when fuel cut is effected while the engine is operated with a rich air-fuel mixture, a response time, from the time the O.sub.2 sensor output indicates mixture richness to the time it shifts to a lean side across a slice level, is measured. Degradation of the O.sub.2 sensor is detected based on the result of the comparison between the response time and a diagnostic reference value which varies in dependence on the engine rotational speed, to thereby improve the determination accuracy. Further, Unexamined Japanese Utility Model Publication No. 2-54347 discloses an apparatus wherein the fuel injection quantity is set using a correction value including a particularly large proportional factor, such that the air-fuel ratio changes stepwise. A time elapsed from the discharge of gas, produced by the combustion of the thus-set quantity of fuel, from the cylinder until the output of an air-fuel ratio sensor crosses a slice level corresponding to the stoichiometric air-fuel ratio is measured. Finally, response degradation of the air-fuel ratio sensor is detected based on the result of the comparison between the measured time and a reference value.
However, improvement in the failure determination accuracy of O.sub.2 sensors is still required.